The present invention relates to a valve for the venting circuit of a liquid tank, in particular a fuel tank with which a motor vehicle may be equipped.
Liquid tanks, in particular fuel tanks for motor vehicles, are nowadays generally provided inter alia with a venting circuit. This circuit allows air to be introduced into the tank in the event of underpressure (especially for compensating for the volume of liquid consumed) or allows the gases contained in the tank to be removed in the event of overpressure (especially in the event of overheating). This circuit also allows the channelling and possible filtering of the gases that have to be discharged into the atmosphere, for the purpose of meeting the ever stricter environmental requirements in this regard.
The venting circuit includes, in a known manner, at least one valve that prevents, as far as possible, liquid from the tank being expelled in the event of the tank being turned upside down or at an excessively high tilt angle. This venting valve must provide a rapid and reliable response when its operating conditions arise, but with minimal sensitivity to transient phenomena such as in particular very high flow rates, overpressure in the tank or low-amplitude waves. It must also ensure that there is minimal liquid carried over into the canister (or the chamber containing a substance, usually activated carbon, which adsorbs the fuel vapours) in normal operation and when filling, for fear of saturating said canister and making the decontamination of the gases discharged into the atmosphere ineffective. This phenomenon is generally called LCO (Liquid Carry Over) in the jargon of the field. Finally, this valve may also carry out the role of vacuum release.
Many venting valves employ a float having an upper needle or tip which closes off an aperture for connecting the tank to the venting circuit. In this type of valve, the functions of vacuum release and LCO limitation may be provided by a suitable geometry of the valve itself or by a separate device, joined to the valve by a section of the ventilation line. Thus, Patent EP 1172306 in the name of the Applicant relates to a float vent valve attached to which is a drainable container (or DC) provided with an umbrella valve capable of opening both to discharge the liquids trapped in the container and in the event of underpressure in the tank.
One problem facing the suppliers of fuel systems to motor vehicle manufacturers is being able to meet the various specifications of these manufacturers with a reasonable range of components. These specifications state quantities such as the ventilation pressure and flow rate; the re-opening pressure of the ventilation valves; the leakage rates tolerated; the possibility of overcoming potential underpressure, etc. which have to be met and which vary from one geographical region to another, and often even from one manufacturer to another.
Regarding the leakage rates, it is important, with the float valves as described previously, that the closure of the aperture is leaktight when the valve is in the closed position. Thus, U.S. Pat. No. 4,982,757 proposes a two-stage valve with double sealing, this being due to an intermediate part that can be moved both with respect to the float and the aperture to be closed off (valve seat) and itself comprising an aperture capable of being closed off by the tip of the float, this aperture being of a smaller size than that of the main aperture (valve seat). This intermediate part comprises an elastomeric part both on its lower and upper faces in order to obtain double sealing (i.e. a leaktight closure both of the main aperture (valve seat) and of the secondary aperture (in the intermediate part)). One drawback of the valves described in that patent lies precisely in the fact that they are specific to a given specification and that, in particular, the dimensions of the apertures and elastomeric parts must be adapted on a case-by-case basis.